Recent studies have shown that hypercapnia and exercise interact synergistically in the control of respiratory ventilation. The synergistic behavior is consistent with a model of the respiratory control system which involves a central optimization process. The underlying hypothesis is that ventilatory output--rather than being set by the sum of chemical and non-chemical inputs as the classical chemoreflex model assumes--may be determined by the balance between the chemical drive to breathe and a propensity of the controller to minimize respiratory effort. The purpose of this investigation is to examine the validity of such an optimization mechanism in ex- plaining the respiratory responses to various disturbances; The aims are: 1. To further characterize ventilatory hypercapnic-exercise interaction under various pathophysiological conditions; 2. To extend the optimization model to include a description of not only the ventilatory response but also all respiratory pattern responses at the neuromuscular level, based upon information gained from the above studies. The ultimate goal is to obtain a general framework from which to understand the mechanisms underlying the control of ventilation, respiratory pattern, and respiratory neural output in health and in pulmonary disease.